Electromagnetic radiation has long been used for surface cleaning. Examples of these processes include the removal of surface contamination, removal of thin material layer coatings such as paints or removal of oils from metal work surfaces. Some of the earliest examples utilized flash lamp radiation sources. These systems can be limited in application because of peak powers achievable.
Lasers have increasingly been used for these types of processes because of the high peak powers achievable, high energy stability and wavelength selectivity. These features allow for high localization, improved material selectivity, and depth control of the cleaning effect. Laser surface cleaning processes can be broadly categorized into surface contamination layer removal and particulate removal. Removal of surface contamination layers is normally accomplished by laser ablation. Particle removal involves removing the contamination as a whole.
Cleaning processes under both categories can benefit from the use of pulsed laser radiation to provide higher peak powers. Short pulsed radiation in particular can provide improved processing. Short pulsed radiation has been shown to decrease the heat affected zone in laser ablation processing. This allows improved localization of the ablative removal as well as finer control of the removal depth. Short pulse radiation also can enhance particulate removal by increasing the rate of thermal increase within the particles and/or substrate thereby increasing the acceleration forces that produce particle removal.
Substrate damage can be an issue for both ablative and particulate removal processes and several techniques have been developed to minimize these effects. For ablative processes, selecting a wavelength that increases the absorption of the contaminant can reduce the fluence requirement and therefore reduce substrate damage. In addition, using multiple pulses for full contaminant removal can reduce the required fluence. However, substrates that have high absorption at the selected wavelengths are likely to be ablated along with the contamination, even with wavelength selection and multiple pulse removal processes. The ability to end stop the removal process at the substrate interface will be limited in these cases. This problem is significantly increased for smaller size contamination, since the absorption cross section for the contamination is reduced relative to the substrate.
As with ablative removal processes, particulate removal processes can also cause substrate damage for sensitive substrates and substrates that have high absorption at the processing wavelength. This problem is increased for small particle removal because of increased adhesion forces between the particles and substrate and self focusing of the laser underneath the particles. For particle cleaning processes, the developed devices and methods for reducing the risk of substrate damage involve controlling the environment above the contaminated surface. Examples of particulate laser processes allowing reduced fluence levels include wet laser cleaning, steam laser cleaning, and increased humidity cleaning. Combinations of laser and other cleaning processes (including etching, organic solvents, and ultrasonic) have been shown to increase cleaning effectiveness and may reduce the risk of substrate damage. However, with the exception of dry laser cleaning processes, all of the particulate removal processes described require access to the environment above the substrate surface. This may be impractical for some systems.
Alternative dry laser particulate cleaning processes have been developed. Laser acoustic wave cleaning and laser shock wave cleaning are dry laser cleaning methods that have also been evaluated for particulate cleaning. Laser acoustic wave cleaning involves direct excitation to the substrate and therefore suffers from a high potential for substrate damage particularly for small particles as discussed. Laser shock wave cleaning has been shown to improve particulate removal and can reduce the risk of substrate damage by focusing the laser above the substrate surface and relying on the shock wave interaction with the particulates. This technique will also have increased difficulty when applied to small particle removal. In addition, the shock wave may damage other sensitive features on or near the surface of the substrate. This is particularly true if there is a sensitive material above the substrate surface, since generating the shock wave requires relatively high laser intensity focused above the substrate.
Even the newest dry laser techniques can also be limited in cases where access to the environment above the surface is not practical (e.g., enclosed systems). The removal process will only move the particle to a different location on the substrate for an enclosed system, because the particles are removed from the surface as a whole. Typically these techniques utilize additional control devices and methods to completely remove the particles from the substrate being cleaned. These methods include directed air flow, use of reduced pressure (vacuum) or gravity most of which require open access to the environment above the substrate surface.
Semiconductor manufacturing is one of the major industrial areas that utilize surface cleaning processes including laser cleaning methods. Many of the required cleaning processes have a stringent tolerance on the allowable level of substrate damage. In addition, the small product features make it necessary to remove very small particles to avoid product failures. Cleaning is an issue in multiple wafer processing steps and includes extended contamination layer (e.g., resist removal) and particulate contamination removal.
Surface cleaning is also a requirement for the optics (e.g., photomasks) used in the wafer manufacturing process. For photomasks in particular, a build up of contamination is observed during the normal usage of the masks in the wafer print processes. These masks are exposed to deep ultraviolet (DUV) radiation during the normal processing used in printing the mask pattern onto the wafer. Exposure to this radiation produces a contamination growth in the form of small particles that absorb the illuminating radiation. This growth is commonly referred to as haze.
Haze formation is a problem for the wafer print process because as the particles increase in size they block more of the light being transmitted through the photomask. Eventually the haze contamination absorbs enough of the light to cause defects in the printed image of the photomask on the wafer. Before the haze contamination reaches this level, the photomask surface must be cleaned. This cleaning requirement has the effect of decreasing the useable lifetime of a photomask because the processes currently used to remove haze deteriorate the absorbing film on the mask. For partial absorbing films, the current cleaning methods reduce the film thickness and therefore affect the films transmission and phase properties. Changes in phase and/or transmission reduce reticle lifetime by altering the size and shape of printed features on the wafer beyond acceptable tolerances. Duplicate sets of photomasks must be made in order to continue manufacturing once the useable life of a photomask is exceeded. Duplicate sets are also required for use while contaminated photomask are being cleaned. There can be a several day time requirement before the mask is cleaned and verified, since the cleaning processes are typically preformed at a different facility. As the required feature size for semiconductor manufacture decreases, the size of haze growth that will produce printed defects also decreases. This increased sensitivity to haze growth means that the newest photomasks will need to be cleaned more frequently and will have a shorter useable life.
Application of alternative cleaning methods to remove photomask surface haze contamination is hindered by the use of pellicles attached to the photomask surface(s). A pellicle consists of a frame that is adhesively bonded to the photomask surface and a thin membrane stretched across the pellicle frame. Pellicles are used to prevent externally generated particles from settling onto the surface of the photomask where they could affect the print process. Externally generated particles settle on the membrane above the mask surface where they have a significantly reduced affect on the print process. With the exception of a small filter valve on the pellicle frame to allow for pressure equalization, the top surface of the photomask is effectively sealed from the local environment by the pellicle attachment.
The current accepted method for haze removal requires the wafer manufacturer to ship the contaminated photomask back to the mask maker or to a third party. Here the pellicle frame is removed from the photomask, the mask is cleaned, inspected for defects and a new pellicle is attached to the photomask and in many cases the mask is inspected again for particle defects before it is shipped back to the wafer manufacturer. This typically takes days to complete, increases the photomask cost due to the additional processing and degrades the photomask quality due to the cleaning process. Additionally, there is a small probability, usually due to the adhesive from the pellicle being removed and falling onto printable areas of the photomask, that the mask will be damaged beyond use by the haze removal process.
Current efforts to improve the issues related to haze growth on photomasks have focused on processes that can be implemented before the pellicle is added because of the difficulties related to through pellicle cleaning. These efforts have been primarily focused on surface preparation and use of alternate chemicals in the cleaning processes. The latter has been shown to change the haze contamination species but not eliminate their growth. Both areas, at best, show a reduction in the growth rate and do not eliminate the requirement for cleaning. More recently, use of an inert environment has been shown to decrease the growth rate of haze formation on photomasks. Application of this method requires control of all environments the photomask is exposed to including all process equipment. As with other methods being developed, this process has the potential to reduce the growth rate but not eliminate the requirement for and the detrimental effects of cleaning.